Improvements in the technology of electrical power generation by wind turbines are being sought throughout the world as part of the effort to reduce dependency on fossil fuels. The European Union has just announced a major sustainable energy project that includes significant use of wind power and is requesting the US to join this effort.
To fully achieve the ultimate potential of such systems, several problems/limitations need to be addressed. First, the family of existing wind turbines share a litany of troublesome limitations including:                (1) Poor performance at low wind speeds, which is most relevant because a majority of the “good-wind” sites have been taken up and the industry has had to begin focusing on technologies for “small wind” sites,        (2) Safety concerns due to poor containment for damaged propellers and shielding of rotating parts,        (3) Irritating pulsating noise that can reach far from the source,        (4) Significant bird strikes and kills,        (5) Significant first and recurring costs due to:                    (i) expensive internal gearing, and            (ii) expensive turbine blade replacements caused by high winds and wind gusts, plus                        (6) Poor and/or unacceptable esthetics for urban and suburban settings.        
The underlying cause for the problems and limitations listed above is that the vast majority of existing wind turbine systems depend on the same design methodology. As a result, virtually all existing wind turbines are unshrouded/unducted, have only a few blades (which tend to be very long, thin and structurally vulnerable) and rotate at very low blade-hub speeds (thus requiring extensive internal gearing for electricity production) but have very high blade-tip speeds (with its attendant complications). These are all similar because they are all based on the same aerodynamic model that attempts to capture the maximum amount of the power available in the wind utilizing the “Betz Theory” for wind turbines (circa 1926) with Schmitz corrections for flow swirl effects, aerodynamic profile losses and tip flow losses. This theory sets the current family of designs and leaves very little room for improving the aerodynamic performance. Thus industry's efforts have primarily become focused on all other non-aerodynamic aspects of the wind turbine, such as, production and life costs, structural integrity, etc.
Wind turbines usually contain a propeller-like device, termed the “rotor”, which is faced into a moving air stream. As the air hits the rotor, the air produces a force on the rotor in such a manner as to cause the rotor to rotate about its center. The rotor is connected to either an electricity generator or mechanical device through linkages such as gears, belts, chains or other means. Such turbines are used for generating electricity and powering batteries. They are also used to drive rotating pumps and/or moving machine parts. It is very common to find wind turbines in large electricity generating “wind farms” containing multiple such turbines in a geometric pattern designed to allow maximum power extraction with minimal impact of each such turbine on one another and/or the surrounding environment.
The ability of a rotor to convert fluid power to rotating power, when placed in a stream of very large width compared to its diameter, is limited by the well documented theoretical value of 59.3% of the oncoming stream's power, known as the “Betz” limit as documented by A. Betz in 1926. This productivity limit applies especially to the traditional multi-bladed axial wind/water turbine presented in FIG. 1, labeled Prior Art.
Attempts have been made to try to increase wind turbine performance potential beyond the “Betz” limit. Shrouds or ducts surrounding the rotor have been used. See, e.g., U.S. Pat. No. 7,218,011 to Hiel et al. (see FIG. 35); U.S. Pat. No. 4,204,799 to de Geus (see FIG. 36); U.S. Pat. No. 4,075,500 to Oman et al. (see FIG. 37); and U.S. Pat. No. 6,887,031 to Tocher. Properly designed shrouds cause the oncoming flow to speed up as it is concentrated into the center of the duct. In general, for a properly designed rotor, this increased flow speed causes more force on the rotor and subsequently higher levels of power extraction. Often though, the rotor blades break apart due to the shear and tensile forces involved with higher winds.
Values two times the Betz limit allegedly have been recorded but not sustained. See Igar, O., Shrouds for Aerogenerators, AIAA Journal, October 1976, pp. 1481-83; Igar & Ozer, Research and Development for Shrouded Wind Turbines, Energy Cons. & Management, Vol. 21, pp. 13-48, 1981; and see the AIM Technical Note, entitled “Ducted Wind/Water Turbines and Propellers Revisited”, authored by Applicants (“Applicants' AIAA Technical Note”), and accepted for publication. Copies can be found in Applicants' Information Disclosure Statement. Such claims however have not been sustained in practice and existing test results have not confirmed the feasibility of such gains in real wind turbine application.
To achieve such increased power and efficiency, it is necessary to closely coordinate the aerodynamic designs of the shroud and rotor with the sometimes highly variable incoming fluid stream velocity levels. Such aerodynamic design considerations also play a significant role on the subsequent impact of flow turbines on their surroundings, and the productivity level of wind farm designs.
In an attempt to advance the state of the art, ducted (also known as shrouded) concepts have long been pursued. These have consistently provided tantalizing evidence that they may offer significant benefits over those of traditional unducted design. However, as yet, none have been successful enough to have entered the marketplace. This is apparently due to three major weaknesses of current designs: (a) they generally employ propeller based aerodynamic concepts versus turbine aerodynamic concepts, (b) they do not employ concepts for noise and flow improvements, and (c) they lack a first principles based ducted wind turbine design methodology equivalent to the “Betz/Schmitz Theory” that has been used extensively for unducted configurations.
Ejectors are well known and documented fluid jet pumps that draw flow into a system and thereby increase the flow rate through that system. Mixer/ejectors are short compact versions of such jet pumps that are relatively insensitive to incoming flow conditions and have been used extensively in high speed jet propulsion applications involving flow velocities near or above the speed of sound. See, for example, U.S. Pat. No. 5,761,900 by Dr. Walter M. Presz, Jr, which also uses a mixer downstream to increase thrust while reducing noise from the discharge. Dr. Presz is a co-inventor in the present application.
Gas turbine technology has yet to be applied successfully to axial flow wind turbines. There are multiple reasons for this shortcoming. Existing wind turbines use non-shrouded turbine blades to extract the wind energy. As a result, a significant amount of the flow approaching the wind turbine blades flows around and not through the blades. Also, the air velocity decreases significantly as it approaches existing wind turbines. Both of these effects result in low flow through, turbine velocities. These low velocities minimize the potential benefits of gas turbine technology such as stator/rotor concepts. Previous shrouded wind turbine approaches have keyed on exit diffusers to increase turbine blade velocities. Diffusers require long lengths for good performance, and tend to be very sensitive to oncoming flow variations. Such long, flow sensitive diffusers are not practical in wind turbine installations. Short diffusers stall, and just do not work in real applications. Also, the downstream diffusion needed may not be possible with the turbine energy extraction desired at the accelerated velocities. These effects have doomed all previous attempts at more efficient wind turbines using gas turbine technology.
Accordingly, it is a primary object of the present invention to provide an axial flow wind turbine that employs advanced fluid dynamic mixer/ejector pump principles to consistently deliver levels of power well above the Betz limit.
It is another primary object to provide an improved axial flow wind turbine that employs unique flow mixing (for wind turbines) and control devices to increase productivity of and minimize the impact of its attendant flow field on the surrounding environment located in its near vicinity, such as found in wind farms.
It is another primary object to provide an improved axial flow wind turbine that pumps in more flow through the rotor and then rapidly mixes the low energy turbine exit flow with high energy bypass wind flow before exiting the system.
It is a more specific object, commensurate with the above-listed objects, which is relatively quiet and safer to use in populated areas.